This invention relates to an improvement on an optical fiber, and more particularly it aims to more effectively dissipate the leaky mode in the cladding by substantially and uniformly sealing particulate substances such as minute hollow glass beads having a refractive index different from that of the primary buffer layer in the primary buffer layer, or on the boundary between the cladding and the primary buffer layer adjacent, to or between the primary and secondary buffer layers to thereby subject the leaky mode to scattering or absorption.
Generally, the fundamental requirements of the primary buffer layer of an optical fiber are; (1) capable of removing the leaky mode; (2) effective as a buffer against lateral pressure; (3) capable of securing the strength of the optical fiber; and (4) can be easily peeled off for connecting operation. Modified silicon, having a refractive index greater than that of the cladding, is usually used as the material for primary buffer. It is known that a buffer, having a refractive index greater than 1.47, is capable of sufficiently removing the leaky mode, and that a marked decrease in strength occurs when the film thickness becomes less than 25 .mu.m, whereas the strength will be stabilized at the film thickness of 35 .mu.m or higher.
A silicon layer having a high refractive index is considered adequate for an elongated optical fiber in terms of leaky mode elimination. However, when the optical fiber is several meters in length, the leaky mode can not be transmitted completely, but instead it is transmitted and partially emitted together with the guided mode from the forward output end of the fiber. Such transmissions of the leaky mode cause fluctuation in the level of output light at the end, distortion in the waveform due to mode dispersion, waver in the plane of polarization and/or deterioration in the polarization, posing difficulties in application of precision measurement systems and optical fiber sensors or in connection to optical devices that are highly dependent on polarization. It is necessary with a single mode optical fiber to align the axes of the cores when two such optical fibers are connected. It is therefore necessary to closely monitor the light leaking in the vicinity of the point where the fibers are connected in abutment so that such leakage of light can be minimized by adequate adjustment of the core axes.
In such a case, the leaking light in the prior art tends to attenuate gradually along the longitudinal direction of the optical fiber. Because the nuclei for scattering are distributed at random due to inconsistency in the refractive index present in the cladding, primary buffer layer and boundary between them the detected level of leaky mode shows fluctuation depending on the location in the longitudinal direction of the optical fiber, as shown in FIG. 1. This makes it difficult to obtain a sufficiently stable level of leaky mode at a predetermined location from the point where the optical fibers are connected in abutment.